There is known a technique called an active noise control (ANC) for making a sonic wave (control sound) having the same amplitude and opposite phase as those of a noise interfere with the noise, thereby controlling the noise by the interference effect. In recent years, there is proposed an active noise control apparatus for an air conditioning noise and an indoor noise in a factory or an automobile and the like.
FIG. 1 is a block diagram of a conventional active noise control apparatus having high noise control performance with a small calculation amount (see Japanese Patent No. 2872545, for example). Here, the conventional technique illustrated in FIG. 1 is called a conventional technique 1.
As illustrated in FIG. 1, a reference signal detecting section 10 disposed in a coming direction of a noise detects a signal (reference signal) concerning a noise generating state, an adaptive filter 20 produces a control signal from the reference signal, and a control sound generating section 30 outputs a control sound based on the produced control signal. A residual noise detecting section 40 disposed in a region where it is desired to control a sound detects a residual noise after the interference, the adaptive filter 20 adaptively obtains a coefficient of a filter which produces the control signal from the reference signal such that the residual noise becomes minimum, so that it is possible to obtain a stable noise control performance which can excellently follow aged deterioration of the control sound generating section 30 and the residual noise detecting section 40 and temperature and humidity changes of a space propagation system from the control sound generating section 30 to the residual noise detecting section 40. The active noise control apparatus having the structure described above is called a feedforward ANC.
Many algorithms such as LMS and RLS have been proposed as adaptive algorithm used here heretofore, but since a control sound is required to be produced in real time, Filtered-X LMS (Least Mean Square) algorithm is frequently used in view of a small calculation amount (see B. Widrow and S. Stearns, “Adaptive Signal Processing” (Prentice-Hall, Englewood, Cliffs, N.J., 1985) and “Active Noise Control”, Corona written by Seiji NISHIMURA, Takeshi USAGAWA, and Shirou ISE). The basic principle is for renewing a filter coefficient based on a steepest-descent method so that the residual noise is reduced in consideration of a transfer function from the control sound generating section to the residual noise detecting section. As illustrated in FIG. 1, if a reference signal at timet 
is defined asx(t),
the reference signal is vectorized to obtainx(t)=[x(t),x(t−1), . . . ,x(t−Nw+1)],
to which a transfer function of an error path from the control sound generating section to the residual noise detecting sectionĉ=[ĉ(1),ĉ(2), . . . ,ĉ(Nw)](wherein,Nw 
is the number of taps of filters of the error path is convoluted to obtain a signal (filter reference signal), the signal is given as illustrated in the equation (1).r(t)=ĉ*x(t)  (1)
(* represents a convolution calculation of vector)
For a renewal equation of filter coefficient, this signal is vectorized to obtainr(t)=[r(t),r(t−1), . . . ,r(t−Nh+1)]
Using this, the renewal equation can be formulated as follows.h(t+1)=h(t)+μ·e(t)·r(t)  (2)Wherein,e(t)
represents, at timet 
a residual noise signal,μ
represents a step size parameter,h(t)=[h(1,t),h(2,t), . . . ,h(Nh,t)](wherein,Nh 
represents the number of taps of the adaptive filter), at timet 
represents filter coefficient of the adaptive filter.
In the conventional technique 1 explained with reference to FIG. 1, when an excessive large control signal is input to the control sound generating section, harmonic distortion or cross modulation distortion is generated due to nonlinearity of a vibration system or a driving system of the control sound generating section (see Japanese Laid-open Patent Publication No. H8-317490, for example).
FIG. 2 is a schematic diagram illustrating that harmonic distortion is generated in a control sound when the excessive large control signal is input to the control sound generating section.
Even if a control signal which is input to the control sound generating section is an undistorted signal as illustrated with a solid line in FIG. 2, when its amplitude is excessively large, a control sound which is output from the control sound generating section becomes a distorted signal having such a shape that a peak portion is slightly crushed as illustrated with a broken line in FIG. 2, and third harmonic illustrated with a chain line in FIG. 2 is included in this distorted signal in addition to the original frequency signal. When original noise exists in the same band as that of the third harmonic, the generation of harmonic deteriorates the sound control effect in the same band as that of this harmonic.
FIG. 3 is a block diagram illustrating another example of the conventional active noise control apparatus (see Japanese Patent No. 3503155, for example). Here, the conventional technique illustrated in FIG. 3 is called a conventional technique 2.
The conventional technique 2 illustrated in FIG. 3 is different from the conventional technique 1 illustrated in FIG. 1 in that a control signal correcting section 50 is disposed between the adaptive filter 20 and the control sound generating section 30. In the control signal correcting section 50, a harmonic is calculated from a control signal which is output from the adaptive filter 20, a correction coefficient is renewed based on a signal in which an error function from the control sound generating section to the residual noise detecting section for the harmonic is convoluted, and a residual noise signal, the harmonic is corrected using the renewed correction coefficient, and the corrected harmonic is added to the control signal which is output from the adaptive filter 20.
Here, the conventional technique 1 explained with reference to FIG. 1 has an adverse effect that if an excessive large control signal is input to the control sound generating section, a harmonic distortion is generated in the control sound due to nonlinearity of the vibration system or the driving system of the control sound generating section, and the sound controlling effect in a band where harmonic is generated is deteriorated. Hence, there is conceived a method in which a signal which cancels an influence of the harmonic distortion is adaptively sought as illustrated in FIG. 3, so that the control signal is corrected, thereby preventing the noise control performance from being deteriorated due to generation of distortion.
The conventional technique 2 illustrated in FIG. 3 has no problem if a harmonic component is correctly estimated and cancelled, but if a frequency component of integral multiple is included in the original noise or a harmonic component is erroneously estimated due to characteristic change of a space transmission system of an error path, there is a problem that not only the adverse effects of the harmonic remains, but also the noise control performance is deteriorated due to the generation of the erroneous counteracting signal.
FIG. 4 is an explanatory diagram of the problem of the conventional technique 2 illustrated in FIG. 3.
Here, it is indicated that noise is generated in two frequency bands before the ANC operation, and the noise in one of the frequency bands is cancelled after the ANC operation, but the noise in the other band corresponding to a harmonic can not controlled sufficiently. When the band where noise is not sufficiently controlled is more important subjectively, the problem is serious.